Re: choosing RAID level for xlogs - Mailing list pgsql-performance

Thanks, everyone. I got some excellent replies, including some long explanations. Appreciate the time you guys took out
forthe responses.
 
 
The gist of it i take, is to use RAID10. I have 400MB+ of write cache on the controller(s), that the RAID5 LUN(s) could
benefitfrom by filling it up and writing out the complete stripe, but come to think of it, it's shared among the two
StorageProcessors, all the LUNs, not just the ones holding the pg_xlog directory. The other thing (with Clariion) is
thewrite cache mirroring. Write isn't signalled complete to the host until the cache content is mirrored across the
otherSP (and vice-versa), which is a good thing, but this operation could potentially become a bottleneck with very
highload on the SPs.
 
 
Also, one would have to fully trust the controller/manufacturer's claim on signalling the write completion. And,
performanceis a priority over the drive space lost in RAID10 for me.
 
 
I can use 4 drives instead of 6.
 
Thanks,
Anjan  

    t-----Original Message----- 
    From: Gregory S. Williamson [mailto:gsw@globexplorer.com] 
    Sent: Tue 8/16/2005 6:22 PM 
    To: Anjan Dave; pgsql-performance@postgresql.org 
    Cc: 
    Subject: RE: [PERFORM] choosing RAID level for xlogs
    
    

    I would be very cautious about ever using RAID5, despite manufacturers' claims to the contrary. The link below is
authoredby a very knowledgable fellow whose posts I know (and trust) from Informix land.
 

    <http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt> 

    Greg Williamson 
    DBA 
    GlobeXplorer LLC 


    -----Original Message----- 
    From:   pgsql-performance-owner@postgresql.org on behalf of Anjan Dave 
    Sent:   Mon 8/15/2005 1:35 PM 
    To:     pgsql-performance@postgresql.org 
    Cc:     
    Subject:        [PERFORM] choosing RAID level for xlogs 
    Hi, 

    

    One simple question. For 125 or more checkpoint segments 
    (checkpoint_timeout is 600 seconds, shared_buffers are at 21760 or 
    170MB) on a very busy database, what is more suitable, a separate 6 disk 
    RAID5 volume, or a RAID10 volume? Databases will be on separate 
    spindles. Disks are 36GB 15KRPM, 2Gb Fiber Channel. Performance is 
    paramount, but I don't want to use RAID0. 

    

    PG7.4.7 on RHAS 4.0 

    

    I can provide more info if needed. 

    

    Appreciate some recommendations! 

    

    Thanks, 

    Anjan 

    

    
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